Undesired vibrations may occur in a wind turbine tower structure. Vibrations can include any kind of oscillatory or repeating displacements in any direction (transverse, longitudinal or torsional) of any amplitude (large or small) and of any frequency (high or low, constant or varying). These vibrations may be caused by different factors, e.g. wind acting on the tower, blades passing along the tower and locally disturbing the wind flow, vibrations transmitted from the gearbox to the tower, rotor movements, nacelle imbalances, vibrations from the hub transmitted to the tower etc. If a tower is subjected to this kind of vibrations during a prolonged period of time, fatigue damage may result. Fatigue damage may lead to a reduced life time of the wind turbine tower and/or its components. Furthermore, the danger exists that when vibrations cause resonance in the wind turbine tower, this can lead to a potentially dangerous increase of the vibrations. As towers become larger and more slender, they also become more sensitive to induced vibrations. Increasing the thickness of the wind turbine tower can overcome these vibrations however, this increase of material in the tower increases the cost of the wind turbine and further complicate the transport of the tower to location.
Wind turbines are being sited offshore, near-shore and onshore. Whether the wind turbine tower is floatingly or fixed-bottom arranged (offshore), arranged on a foundation in the sea bed (near-shore), or placed on a soil foundation (onshore), the waves of the sea and violent winds may form another source of vibrations in the wind turbine tower. Additionally, the design tip speed ratio of wind turbines placed offshore or near-shore is generally higher than for wind turbines placed on shore. The hub will thus rotate at higher speeds. The frequency with which blades pass past the tower thus also increases. The danger of vibrations reaching a resonant frequency of the wind turbine tower increases therewith